Hi Dean,
I'm not sure it would be worthwhile translating one iDoc into another. With probably less complexity, it might be better to define a separate Process Code and build a new Function Module to process the CCECUS iDoc data into the ECC_CTAX_S structure - a hybrid of Function Modules /SAPSLL/IDOC_INPUT_CCECUS and /SAPSLL/IDOC_INPUT_CTAX.
Provided you can separate the two sets of data (perhaps by having the Broker send to different addresses of including some identifier in the message header), you should then be able to process them separately and correctly.
On the other hand, you'll probably find that once the Broker "gets the hang of" transforming his data into an iDoc format, he'll pretty quickly be able to produce the second mapping once he's done the first. So he may quote you 20 days development for the CCECUS iDoc, but then only need 2 more days to produce the CTAX version.
To answer your final direct question; the iDoc definitions are in the system - Transaction WE60 holds them.
Hope that helps.
Regards,
Dave